Nonlinear dynamics and synthetic-jet-based control of a canonical separated flow

Abstract

A novel flow configuration devised for investigation of active control of separated airfoil flows using synthetic jets is presented. The configuration consists of a flat plate, with an elliptic leading edge and a blunt trailing edge, at zero incidence in a free stream. Flow separation is induced on the upper surface of the airfoil at the aft-chord location by applying suction and blowing on the top boundary of the computational domain. Typical separated airfoil flows are generally characterized by at least three distinct frequency scales corresponding to the shear layer instability, the unsteadiness of the separated region and the vortex shedding in the wake, and all these features are present in the current flow. Two-dimensional Navier-Stokes simulations of this flow at a chord Reynolds number of 6 104 have been carried out to examine the nonlinear dynamics in this flow and its implications for synthetic-jet-based separation control. The results show that there is a strong nonlinear coupling between the various features of the flow, and that the uncontrolled as well as the forced flow is characterized by a variety of lock-on states that result from this nonlinear coupling. The most effective separation control is found to occur at the highest forcing frequency for which both the shear layer and the separated region lock on to the forcing frequency. The effects of the Reynolds number on the scaling of the characteristic frequencies of the separated flow and its subsequent control are studied by repeating some of the simulations at a higher Reynolds number of 1 105. © 2010 Cambridge University Press

Effect of trailing edge shape on the separated flow characteristics around an airfoil at low Reynolds number: A numerical study

“…For example in the experiments of Yarusevych et al 52 , for the range of Reynolds numbers for which reattachment occured, this ratio was at least 7. There is one study in which these two frequencies are similar, that of Kotapati et al 16 . The authors studied a flow configuration with laminar separation, reattachment and vortex shedding.…”

“…Their 2D simulations showed that the shear frequency and the shedding locked to a single frequency of 2.9, while we find locking at a larger frequency (equal to 4.5). The explanation for this difference is the following: the authors induced the separation bubble close to the trailing edge (the reattachment point is located at 0.97C), therefore the effective length scale for the vortex shedding (in essence, the effective thickness seen by the flow) is the sum of the plate thickness and the height of the separating bubble 16 (equal to 0.037C and located at around 0.88C, as can be observed from their figure 5). In our case the separation bubble appears in the middle of the airfoil, the flow reattaches at 0.56C, so the thickness of the bubble does not affect the effective length scale of shedding.…”

A direct numerical simulation investigation of the synthetic jet frequency effects on separation control of low-Re flow past an airfoil

“…[28][29][30] The forcing frequency of the SJ is arguably the most important parameter and is indeed the key parameter to be optimized for better separation control effect. 24,[31][32][33] Experimental and numerical results 31,34,35 reveal that the fundamental mechanism of separation control by a SJ is the formation of coherent roll-up lifting vortices not far from the airfoil surface. Such vortices are also observed in the cases of a nominally 2D hump 17 and flat plate.…”

Synthetic Jet Actuator-Based Aircraft Tracking Using a Continuous Robust Nonlinear Control Strategy

“…Various approaches have recently been developed for aircraft tracking control using SJAs (e.g., see [8,[10][11][12][13]), where the SJA actuator uncertainty is compensated using adaptive control methods or neural networks. Other popular approaches for SJA-based control are computational fluid dynamics-(CFD-) based numerical techniques (see [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]). Adaptive control, neural networkbased control, and numerical CFD methods have been shown to be effective in their respective SJA-based control tasks.…”